Fix bug #65088 (Generated configure script is malformed on OpenBSD).

We fell foul of a "common urban legend"[0], which resulted in us backslash
escaping double quotes that we didn't need to in acinclude.m4. This worked fine
on most shells, but OpenBSD's ksh implementation really didn't like it.

[0] http://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.69/html_node/Shell-Substitutions.html